To determine the infectivity and maturity of metacercariae of Paragonimus westermani after keeping at low temperature for a long period, 45 mice and 45 rats were each infected with 20 metacercariae which were kept at 4 degrees C for 8 to 234 days. The worm recovery in mice increased with age of worm and reached a peak of 32% at 41-50 days and then decreased with age. The rate in rats first decreased to a lowest point of 6% at 71-100 days and then increased with age. In 42 infected mice and 41 infected rats, 187 immature worms (183 tiny and 4 juvenile ones) and 190 worms (164 tiny, 19 juvenile and 7 mature ones) were recovered respectively. Two worm cysts with eggs only and 8 empty worm cysts were also found in the rats. In addition, the frozen metacercariae can still develop to mature worms in SD rats.
